1. Field of the Invention
The present invention relates to a motor drive circuit, a fan motor, an electronic device, and a notebook personal computer.
2. Description of the Related Art
In an electronic device such as a notebook personal computer, a fan motor is used to cool down a heat generating part such as a processor. Although the cooling performance can be improved by maximizing the rotation speed of the motor when the fan motor is used to cool down a heat generating part, the rotation speed of the motor must be adjusted in accordance with a heat generation amount for reducing power consumption and noise. For example, the rotation speed of the motor can be adjusted by increasing or decreasing a drive voltage applied to the motor coil in accordance with the heat generation amount.
To further enhance the reduction of power consumption and noise, intermittent drive is performed in addition to the control of the drive voltage. For example, Japanese Patent Application Laid-Open Publication No. 2006-174648 discloses a mode of reducing a rate of application of the drive voltage to the motor coil as the target rotation speed of the motor is lowered while performing control such that the drive voltage is reduced in accordance with a target rotation speed of the motor.
By reducing a rate of application of the drive voltage to the motor coil as the target rotation speed of the motor is lowered, the rotation speed of the motor can be controlled to a lower rotation speed as compared to the case of control only with adjustment of the drive voltage, and the reduction of power consumption and noise can be achieved.
By the way, when the motor rotates, a cogging torque is generated due to the attractive force and repulsive force in accordance with relationship between positions of magnetic poles and a position of the motor coil. While the motor is stopped, the motor coil is often located at a position causing the lowest cogging torque. Therefore, a torque must be greater than the cogging torque to start rotation from the stopped state of the motor.
In the mode disclosed in Japanese Patent Application Laid-Open Publication No. 2006-174648, if the rotation speed of the motor is lowered, since the drive voltage is low and the rate of the drive is reduced, the torque for driving the motor is also reduced. Therefore, although the motor can continuously be rotated with a small torque due to the effect of inertia while the motor is rotating, when the rotation is started at a low speed while the motor is stopped, the motor may not be activated at low speed since the torque driving the motor cannot exceed the cogging torque. Especially, in the case of single-phase motors, the activation at low speed is likely to be difficult since a difference is larger between the maximum level and the minimum level of the cogging torque as compared to three-phase motors.